A major goal of our research is to understand the physiological mechanisms that regulate brown and beige fat activity. Beige fat cells, in particular, are associated with protection against obesity and metabolic disease. There is thus great interest in understanding the mechanisms that regulate beige fat development and function. Beige fat cells develop from a population of PDGFR?+ fibro-adipogenic progenitor (FAP) cells that can also differentiate into myofibroblast-like cells and promote fibrosis. We found that the brown fat cell regulator, PRDM16 plays an important role in suppressing the fibrogenic activity of FAPs cells during the WAT beiging process. Our results suggest that PRDM16 blocks the pro-fibrogenic effects of Hypoxia-inducible factor-1? (HIF1?), an adaptive regulator of the hypoxia response, which is activated during the beiging process. Genetic or age-related reduction of PRDM16 led to a dramatic impairment of beige fat adipogenic potential and a reciprocal increase in adipose fibrosis. Cell culture studies suggest that PRDM16-expressing (beige) adipocytes act in a paracrine manner to suppress HIF1? functions in FAP cells. Specifically, PRDM16 drives the production of the ketone body, ??-hydroxybutyrate (BHB). BHB is sufficient to promote beige adipogenesis and reduce myofibroblast differentiation in FAPs by inhibiting the function of Hypoxia-inducible factor-1? (HIF1?). The activity of BHB in regulating adipose progenitor differentiation requires the ketone-metabolizing enzyme BDH1. Our central hypothesis is that BHB-catabolism in FAPs plays a critical role in promoting beige fat differentiation and reducing adipose fibrosis. We posit that raising ketone body levels may partly replace PRDM16 function in blocking myofibroblast differentiation. Finally, we hypothesize that cold-induced HIF1? activity drives adaptations that are essential for brown/beige fat thermogenesis. We will use a combination of mouse models, metabolomic techniques and physiological assessments to address these hypotheses. Specific Aim 1 uses stable isotope tracer metabolomics to evaluate the effect of HIF1? and BHB catabolism on metabolic pathways and metabolites involved in the epigenetic regulation of gene expression in adipose progenitors. This aim will also identify the specific gene targets of HIF1? and BHB. Specific Aim 2 examines the in vivo physiological role of BHB metabolism in beige fat development and function. This includes approaches to determine if raising BHB levels has therapeutic potential to reduce age-related fibrosis and metabolic disease. Specific Aim 3 is to determine the physiological role of HIF1? and its interaction with PRDM16 in controlling brown/beige fat thermogenesis using new genetic mouse models. This work will provide important insights into the novel role of metabolism and metabolites in brown/beige fat thermogenic remodeling. We anticipate that this will also suggest new approaches to increase brown fat mass and/or reduce fibrotic disease.